1. Field of the Invention
The invention relates generally to the field of drilling Earth formations below the bottom of a body of water. More specifically, the invention relates to remotely operated drilling devices that are positioned on the sea floor.
2. Background Art
Drilling through Earth formations located below the bottom of a body of water generally require the use of drilling equipment deployed from a barge or ship, and in the case of deep water sites, from a drillship or semisubmersible floating drilling platform. Such drilling is a complicated and expensive operation, particularly in deep water where a drilling riser must be extended from the floating drilling structure to the sea floor to provide a return conduit for drilling fluid from the well as it is drilled. In addition to cost, drilling using such riser is not well suited to drilling tasks requiring precise control of bit weight, stability (motion compensation) of the drill string and exact positioning of tools within the borehole. Positioning of the surface vessel over the borehole on the seabed is of critical importance when a drilling riser is used. Multiple anchors or dynamic positioning are required to maintained the required degree of positional stability of the floating drilling platform. The in-water weight of the riser limits the water depth in which risers can be deployed. Floating drilling platforms capable of handling long risers for deep water are by necessity very large vessels.
In an attempt to minimize the above noted aspects of drilling in deep water, several seafloor based drilling systems have been developed and are in current operation. “BMS #1” and “BMS #2” are owned by JOGMEC (Japan), “PROD” is owned by Benthic Geotech Pty. Ltd. (Australia), “MeBo” is owned by the University of Bremen (Germany) and “RD2” is owned by the British Geologic Survey. The forgoing remotely operated systems have proven effective in drilling into the seabed, particularly in deep water. Because they all use a flexible umbilical rather than a drilling riser, the in-water weight of such systems is typically less than 20 tons and as a result drilling operations can be conducted from vessels as small as 50 m in length. Station keeping (positional stability) requirements for the vessel are much less stringent than for floating drilling platforms using riser, and an operational watch circle of about 20% of the water depth is adequate in most cases. Because the drilling systems are disposed on the water bottom while drilling and are necessarily heavy enough to provide sufficient reactive mass to advance the drill string, the stability of tools disposed within the borehole is excellent. Complete decoupling of drill string motion from ship motion is accomplished.
When used to drill core samples of the subsurface below the bottom of a body of water, all of such remotely operated water bottom drilling systems depend upon rod coring methodology. A core barrel is disposed at the bottom of a drill string. The core barrel is typically about the same length as one segment of drill pipe or string. As the borehole is extended by drilling, the core barrel is filled and then must be retrieved from the borehole to extract the core therein. Such methodology requires the retrieval of the entire drill string each time a core barrel is recovered. While the foregoing method operated from a water bottom disposed drilling unit eliminates the drill pipe riser extending from the floating drilling platform to the water bottom, the extensive tool handling required by such coring techniques results in a significant operational time to complete boreholes deeper than about 30 meters. A single 100 meter deep borehole using rod coring with standard 3 meter core barrels and drill rods requires more than two thousand tool handling operations and over one hundred hours complete. The extensive time on station and the large number of tool manipulations make rod core drilling impractical for all but shallow holes in deep water.
There exists a need for a water bottom based drilling unit that can obtain core samples with reduced tool handling an operating time.